The invention relates generally to systems and methods for supporting a workpiece in a position for inspection or other processing, and relates more particularly to levitating the workpiece with a defined pattern of levitation forces along the surface area of the workpiece.
There are a wide variety of manufacturing applications in which the detection of surface defects and irregularities plays an important role in determining manufacturing yield. As examples, semiconductor substrates on which integrated circuits are fabricated and masks that are used during the fabrication process should be free of particles, such as dust. As another example, glass used in the fabrication of flat panel displays and similar applications should not include surface defects such as bumps, pits and scratches.
Surface defects and irregularities can be optically detected. As one example, U.S. Pat. No. 5,459,576 to Brunfeld et al. describes an optical inspection system that includes a spatially coherent light source to illuminate the surface of a workpiece to be inspected. A single beam of laser light is modified to produce what is referred to as a xe2x80x9cblack beam.xe2x80x9d A xe2x80x9cblack beamxe2x80x9d is defined as a beam that has an intensity of zero in the vicinity of the optical axis. The result is that when the undisturbed beam is detected by a detector close to the optical axis, the detector output is zero. However, if the beam encounters a defect, the balance is upset and the detector will register a non-zero output. This non-zero output provides quantitative information regarding the defect or irregularity.
An optical system for inspecting glass workpieces is described in U.S. Pat. No. 5,726,749 to Schave. A number of laser light sources are positioned on one side of the sheet of glass or other transparent material. A corresponding number of photosensitive position detectors are located on the opposite side. Light beams are directed from the laser sources through the transparent sheets to the position detectors. The angular deviation of each light beam which occurs as the light beam passes through the sheet affects the signal at the corresponding position detector. The signals generated as a result of angular deviation of the light beams from their optical axes are processed to provide information regarding the angular deviation values of the transparent sheet. The distortion is determined by the rate of change of the angular deviation and is calculated for comparison to preset standards, so as to determine if the transparent sheet is within acceptable quality control measures. The inspection system of Schave includes a conveyor for transporting the transparent sheets relative to the laser sources. The transparent sheets are supported on motor-driven belts.
While the use of motor-driven conveyor belts for optical inspection systems may provide acceptable results in some applications, other applications require a level of measurement preciseness that may not be achievable using standard mechanical approaches. An alternative approach is to use air pressure to levitate the workpiece to be inspected. A flat panel display may be more precisely inspected while being levitated by an air film.
A conventional air table for supporting a workpiece on an air film includes a work surface having an array of through holes. Pressurized air escapes from the through holes to apply a force to the workpiece. In some applications, the flatness of the workpiece may be affected by differences in the localized pressures applied to the workpiece. Laser scanning will detect very small changes in angular distortion of a flat panel display or other product that is susceptible to some bending as a result of non-uniformity in applied localized pressures.
A concern is that equalization of the air flow through the holes of the air table may not result in uniformity of localized pressures. For a conventional air table, the pressurized air escapes from the array of through holes to strike the workpiece and then leaks from the edges of the workpiece, unless the workpiece allows the passage of air. For a solid workpiece, the result is that the pressure is greatest at the central region of the workpiece and least at the edges, where the air is allowed to escape than at its edges. This may cause a xe2x80x9cbulgexe2x80x9d in the center region of the workpiece. This bulge will be detected as a surface irregularity during a laser scanning operation. Moreover, as consequences of the limited depth of field of detection optics, tangential error may cause the laser beam to miss the detector and DC error may occur.
What is needed is a system and method for levitating a workpiece in a manner that reduces susceptibility to angular distortions as a result of variations in localized pressures and overall pressure gradients.
Workpiece planarity is promoted by utilizing a support structure having reverse flow exhaust openings interspersed with supply openings that direct a levitating flow of fluid to impinge the workpiece. Rather than focusing on regulating the flow of supplied fluid to the fluid film region between the support structure and the workpiece, the focus is on regulating the air leakage path from the fluid film region. Specifically, lateral fluid flow to the edges of the workpiece is retarded. In the preferred embodiment, the work surface of the support structure includes planar raised regions about the exhaust openings, so that fluid flow is regulated on the basis of closure distance, in the same manner as a pinch valve. Also in the preferred embodiment, the fluid is a gas, such as air.
The air distribution supply holes are clustered around exhaust holes to provide localized cells of air flow at equilibrium conditions. Overall sizing and the associated pressure differentials through the holes are determined as a function of the overall system design and as a function of the need for all pressure cells to be operating with similar performance across the entire surface of the workpiece being supported. Additionally, it is often the case that there are flow cells that are uncovered as the workpiece is transported over the work surface. The supply holes and exhaust holes are therefore open to atmosphere and must be small enough so as to not degrade the performance of the cells still actively supporting the workpiece. Larger holes require a larger flow source and larger manifolds for distribution. Higher xe2x80x9cflying heightsxe2x80x9d are a result of this increased flow.
In the operation of the method, gas is continuously fed through the supply openings. When the workpiece is positioned adjacent to the work surface of the support structure, the supplied gas pressure supports the workpiece. The combination of atmospheric pressure on the workpiece and reverse flow through the exhaust openings quickly causes the workpiece to drop close to the support structure. As a result, the workpiece approaches a position in which it blocks the exhaust openings, particularly in the embodiment in which the exhaust openings are within raised regions along the surface of the support structure. An equilibrium condition is soon established in which the gas supply volume equals the gas leakage volume. After the equilibrium condition has been reached, any external forces which tend to press the workpiece will cause an increase in the applied pressure. Conversely, any external forces which tend to separate the workpiece from the support structure will cause a pressure drop. The result is a self-managed xe2x80x9cflying height.xe2x80x9d Depending upon the workpiece, the approach may operate to flatten curled or distorted material. For example, the workpiece may be a continuous web of flexible material.
Preferably, the support structure is non-particulating. The support structure may be a substantially planar xe2x80x9ctablexe2x80x9d formed of a high density polypropylene, but other materials (e.g., aluminum) may be used. The work surface of the table may be white if the system is to be used in a clean room or may be black in order to facilitate inspection using the xe2x80x9cblack beamxe2x80x9d technology. The support structure may include a manifold arrangement in which the supply openings are connected to a source of pressurized gas, while the exhaust openings are connected to a vacuum source. Optionally, a single pump may be used to provide both the gas pressurization for the supply openings and the partial vacuum condition for the exhaust openings. The capacity of the exhaust should exceed the flow of gas from the supply, so that lateral flow to the edges of the workpiece is retarded.
In operation, the supply arrangement provides the lift, while the exhaust arrangement regulates the height of the supported workpiece. Pressure from the supply openings will properly support a load if there is sufficient resistance to lateral flow to develop pockets of lifting pressure. The raised regions about the exhaust openings regulate the height by functioning as pinch valves. That is, the distances between the workpiece and planar upper surfaces of the raised regions will regulate the characteristics of the pockets, since these distances determine the flow rate into the exhaust openings.
The size of the support structure will depend upon the application. Contemplated applications include inspecting glass for flat panel displays, semiconductor wafers for the integrated circuit chip industry, and webs of flexible material. For the glass substrate application, the thickness of the glass may be in the range of 0.3 mm to 10 mm, but this is not critical. A flying height may be in the range of 0.05 to 0.5 mm.
While not critical, the supply openings may have a cross sectional area that is smaller than the cross sectional area of the exhaust openings. For example, the supply openings may have a diameter of 0.16 mm, while the exhaust openings may have a diameter of 0.8 mm. However, the supply openings preferably outnumber the exhaust openings.